1. Technical Field
The present invention relates generally to a power factor correction circuit, and more particularly to an AC/DC converter with passive power factor correction circuit and a method of correcting power factor.
2. Description of Related Art
Typically, an AC/DC converter is used to convert an alternate current (AC) into a direct current (DC). A conventional AC/DC converter is shown in FIG. 1, which includes a rectifier circuit 10 and an output capacitor C, wherein the rectifier circuit 10 converts an alternate current (AC) provided by an AC power supply S into a direct circuit (DC), and the output capacitor C bridges over the output of the rectifier circuit 10 and are in parallel with a loading R. While the AC/DC converter is working, the phases of the input voltage vin and the input current iin of the AC are different, which leads to low power factor and poor total harmonic distortion. In addition, the output capacitor C is charged only when a voltage of the DC is higher than that of the output capacitor C, hence the charging time of the output capacitor C is shortened. As a result, the conduction time of diodes in the rectifier circuit 10 is also shortened, which increases the peak value of the conduction current, distorts the waveform of the input current iin, and lowers the power factor. Low power factor not only wastes energy, but also poses unnecessary burdens for the power supply systems of the power company. Therefore, the AC/DC converter with power factor correction circuit is emerged.
In general, there are two types of the conventional power factor correction circuits of the AC/DC converters, which are active type and passive type. The active power factor correction circuit controls the input current with active switch components, and this type of the power factor correction circuit has several advantages, such as the power factor could excess 0.99, and the total harmonic distortion could be less than 10%. And moreover, the active power factor correction circuit is compatible with a wider range of input voltage, generates a stable output voltage, and it's unaffected by the variation of output power. However, the active power factor correction circuit has several drawbacks too, such as higher cost due to additional active switches, high electromagnetic noises, and low durability. Please refer to FIG. 2, the conventional passive power factor correction circuit has an inductor L in serial with the input of the rectifier circuit 10 of the AC/DC converter. Since the structure of the passive power factor is simple, it is more durable. And there is no active switch which causes electromagnetic noises needed. Because the inductor L has to be made from a bulk of silicon steel plate, its size increases along with the decreasing of output power and rated input voltage, and the power factor could merely achieve around 75%, which is insufficient to meet the demands nowadays. If the power factor of passive power factor correction circuits of AC/DC converters could be increased, those AC/DC converters which have more expensive active power factor correction circuits could be replaced. Besides, in order to reduce the ripple of the output voltage of the loading R, the aforementioned output capacitor C has to adopt a high capacity electrolytic capacitor, but the electrolytic capacitor is easy to leak out the contained electrolyte due to being heated for a long period of time, which shortens life of circuit.